In testing and improving sports balls such as basketballs, baseballs, softballs, tennis balls, and golf balls, it is important to understand the impact behavior of the ball. For example, to understand the impact behavior of a golf ball when struck by a golf club, the impact can be modeled using Finite Element Analysis (FEA). Reliable FEA modeling requires accurate numerical characterization of the nonlinear viscoelastic properties of elastomeric materials at moderately high strains, such as xcex5max≈0.3, and high strain rates, such as dxcex5max/dt≈103secxe2x88x921. This requires measurement of stress/strain for sample specimens undergoing such deformations. Conventional hydraulic testers can easily achieve the required strains. Piezoelectrically driven vibrometers or shaker tables can easily achieve the required strains, but not high strain rates and are not well-suited to a single impact event. The present invention is directed toward a striker device capable of achieving the desired strain and strain rates.
As noted above, it is known in the art to use hydraulic devices and piezoelectrically driven vibrometers to achieve strains in test materials. The Ferguson U.S. Pat. No. 5,092,179, for example, discloses a material testing system in which a hydraulic ram compressively deforms the specimen and permits specimen deformation and strain rate to be independently controlled. Similarly, the Keener et al. U.S. Pat. No. 5,677,494 discloses a method for high strain-rate testing of specimens using an incremental mechanical loading apparatus.
Impact testing for golf clubs, shafts and the like is disclosed in the U.S. Pat. No. 5,739,411 to Lee et al. A compressed spring is used to drive a hammer against one end of a golf shaft under test. A load cell is used to provide an impact force measurement.
While the prior devices operate satisfactorily, they are not intended for nor capable of measuring strain in an elastic material in response to impact as is important for sports ball analysis.
Accordingly, it is a primary object of the invention to provide a device for measuring the force and resulting deformation of a sample of material, preferably an elastomeric material, used in sports balls when the force is applied so as to produce a high strain rate in the material. The device includes a base on which the sample is mounted, a striker for delivering an impact to a surface of the sample in a first direction, a load cell for measuring the reaction force of the impact from the sample and an optical detector for measuring displacement of the sample in the first direction. The reaction force and displacement measurements are processed to determine the time dependent strain of the material.
According to a more specific object of the invention, the sample is sandwiched between a lower platen such as a bolt passing through the base and having a head portion on which the sample is placed, and an upper platen. The load cell has an annular configuration and surrounds the bolt shaft below the head and above the top of the base. The striker is dropped on the upper platen and the reaction force from the sample is measured by the load cell.
It is a further object of the invention to provide a plurality of optical detectors around the base. The detectors direct light beams against the underside of the upper platen so that deflection of the head resulting from displacement of the sample upon impact can be detected.